The invention relates to a process for the detection of a product in the discharge of a catalytic material of a plurality of catalytic materials which are exposed to a reaction gas.
The development of new, especially more effective catalysts is required to improve the yield of many standard processes. In order to produce and characterise catalysts, the potentially active carrier and catalyst components are combined in a supposedly suitable manner and the multi-component mixtures thus created are tested with a suitable test reaction under defined conditions usually determined by technical limitations. Whilst the synthesis of such materials can often still be managed with reasonable time consumption, the testing of the catalysts on the contrary represents a very time- and labour-consuming step. The material to be tested is generally poured into a laboratory test reactor specially designed for the purpose and tested under prescribed parameters for its suitability for the given conversion of an educt mixture.
Gas chromatographic or spectroscopic methods are chiefly used to characterise catalysts. These methods, however, have only a limited suitability for the simultaneous testing of many catalysts. Whereas gas chromatography has the drawback that it requires a certain period of time, the spectroscopic methods have the drawback that, in the case of complicated reactions, they give insufficient information about the selectivities of the catalysts. Both methods also have the drawback that they only function sequentially and involve an elaborate and costly analysis.
Nonetheless, progress has been made with the applicability of both these methods. Through the computer-controlled combination of several columns, for example, an improvement has been achieved in the analysis quality and the analysis speed in gas chromatography (Stockinger, J. H., Callen, R. B., Kaufman, W. E., J. Chromatogr. Sci. 16, 1978, 418).
The use of spectroscopic methods for characterising samples is described in connection with a process for investigating chemical reactions in reactors connected in parallel (Windhab, N., Miculka, C., Hoppe, H.-U., DE 196 32 779).
A method of testing catalyst pellets with the aid of infrared thermography has also been published (Moates, F. C., Somani, M., Annamalai, J., Richardson, J. T., Luss, D., Willson, R. C., Ind. Eng. Chem. Res. 35, 1996, 4801). This method, however, is limited to reactions with great heat of reaction. An additional drawback with this method lies in the fact that, with partial oxidations for example, the hottest catalyst is not the one with the greatest selectivity for the desired product.
WO 97/32208 describes a catalyst testing process, in which a plurality of differently composed catalysts in the form of pellets or suchlike are arranged in a multi-cell holder and brought into contact with an inflow of a mixture. The measurement takes place spectroscopically, thermographically or with the aid of other commonly used measuring methods, whereby the measurement is always carried out directly on the catalysts.
DE 196 33 779 A1 describes a process for investigating chemical reactions in miniature reactors connected in parallel. The supplied educt is led past a catalyst in each case, as a result of which a reaction mixture arises which in turn is passed in each case into a cell drill-hole and separated out via the latter. For the measurement, an infrared beam extends in each case through one of the cell drill-holes which are arranged in the manner of an nxn matrix.
In the light of the prior art, the problem of the present invention is to make available an economically practicable process for the detection of a product in the discharge of a catalytic material of a plurality of catalytic materials which are exposed to a reaction gas, in order to establish the selectivity and activity of the catalytic materials.
Furthermore, the problem of the invention is to provide a method for the testing of catalytic materials that is as cost-effective, simple and efficient as possible.
The process should further be able to be carried out without taking a great deal of time and effort.
These problems, as well as other problems not explicitly stated but which can readily be derived or inferred from the context discussed herein, are solved by the measures described in claim 1. Expedient modifications of the process according to the invention are protected in the sub-claims related back to claim 1.
By disposing an adsorbent selective for the product to be determined in the discharge of each catalytic material, which changes at least one of its properties through the contact with the product to be determined, and then by ascertaining the change in the property of the adsorbent, it is possible to ascertain the selectivity and activity of the catalytic materials.
On the one hand, the process is cost-effective and efficient, and on the other hand the process can be carried out without taking a great deal of time and effort.
The arrangement for the analysis can be spatially separate from the catalytic material. The analysis of the products can thus be carried out by means of methods that cannot be used under the reaction conditions of the catalytic conversion, because for example the adsorbent is not stable at the temperatures at which the catalysed reaction must take place. If the adsorbent is not destroyed by the selected reaction conditions, it can however also be spatially very close to the catalytic material.
By means of a suitable arrangement of various adsorbents, different products of the catalytic reaction can be determined simultaneously. It is thus possible in a very elegant manner to determine, apart from the activity of a catalytic material in respect of a desired product, also the concentration of undesired by-products, so that the selectivity of the catalytic material can also be determined.
Preferred forms of embodiment of the process according to the invention are characterised by the fact that the adsorbent is a solid or a liquid applied to a solid carrier.
Preferably, an optical property, especially in the infrared, visible or ultraviolet region, is changed by the contact with the product to be determined.
It is particularly preferable for a colour change to occur through the contact of the adsorbent with the product of the catalytic conversion to be determined, so that for example a colour formation or a decolourisation can be ascertained.
It is further preferred that the property of the adsorbent changing through the contact with the product to be determined is a fluorescence.
A further property that can be changed by the contact with the product to be determined is the weight of the adsorbent.
A further preferred modification of the process according to the invention employs the temperature of the adsorbent to determine a product of the catalytic reaction.
A further property of the adsorbent that can be changed by the contact with the product to be determined is the refractive index of a liquid applied to a solid carrier.
Furthermore, the pH of the adsorbent is a property that can be changed by the contact with the product to be determined, especially when the adsorbent is a liquid applied to a solid carrier.
It is particularly preferable for a change in the conductivity of the adsorbent to occur through the contact with the product to be determined.
The product to be determined can also be dissolved out of the adsorbent with the aid of a solvent, after which a property, such as for example the refractive index, the pH value and the conductivity, of the solution thus obtained is determined.
Preferred forms of embodiment of the process according to the invention are characterised in that the product to be determined is maleic anhydride and the adsorbent is an N,N-dimethylindoanyline applied in ethanol solution onto a filter paper, whereby the colour of the adsorbent changes from blue to colourless upon contact with the maleic anhydride.
In a preferred form of embodiment of an adsorbent described above, reactive groups capable of reacting selectively with a desired product in the discharge of a parallel reactor are applied to a solid carrier.
In a further preferred form of embodiment, reactive molecules capable of reacting selectively with a desired product are dissolved in a solvent, after which the latter is applied to a solid carrier.
Detection of a product should be understood within the scope of the present invention to mean any identification of a chemical element, a chemical compound or a functional group which are formed in a catalytic reaction.
It can relate here, for example, to a compound whose formation is intended to be optimised by the catalytic conversion.
It is however also conceivable for a compound to be determined that necessarily arises with the formation of the desired product. Amongst others, this includes for example hydrogen which is liberated in a dehydrogenation.
By means of the process according to the invention, however, substances can also be detected whose formation is to be avoided, i.e. by-products of a catalytic conversion.
A catalytic material is a substance which lowers the activation energy for the course of a certain reaction and thus increases the reaction rate, without appearing in the end product of the reaction. Substance is understood here to mean for example elements and compounds as well as homogeneous and heterogeneous mixtures thereof. Binary or ternary mixtures of elements and/or compounds in particular often display an unexpectedly high catalytic effect which can only be ascertained empirically.
The process according to the invention makes it possible to investigate simultaneously under identical reaction conditions many catalytic materials which differ in their composition. The essential feature of the invention here is that the discharge of a catalytic material can be assigned to a change in the property of the adsorbent. It may be necessary here for the discharges of the respective catalytic materials to be kept separate from one another. The plurality of catalytic materials, i.e. at least two, can for example be introduced into parallel reactors.
Discharge is understood to mean the gas mixture after the contact with a catalytic material, which can contain at least one reaction product. The reaction gas is a gas that contains the reactant or reactants. It can in addition contain inert gases, such as for example nitrogen or noble gases (in particular helium, neon, argon or mixtures of these gases), in order for example to improve the conducting of the reaction.
Reactant is understood here to mean the starting substance or starting substances of a catalytic conversion. In a dehydrogenation, it can for example be an alkane, such as ethane. In other reactions, such as for example a hydrogenation or a partial oxidation, at least two reactants are required, such as for example hydrogen and an alkine in a hydrogenation, or oxygen and an alkane, such as n-butane in a partial oxidation.
Selectivity is the ability of the adsorbent to select preferentially one of a number of presented compounds in the discharge of the reaction gas.
Adsorbent designates a substance which is capable of concentrating at its boundary surface or absorbing into itself certain substances from gaseous mixtures, which can be linked to an increase in volume. The adsorbent can be a solid or a mixture of solids, such as for example activated carbon, aluminium oxide, silica gel, soot, zeolite, or liquid that is applied onto a solid carrier, such as for example an ethanol solution of N,N-dimethylindoanyline, which is applied to filter paper. These substances can also be used as a mixture. It is possible to provide the adsorbent with reactive groups or to load it with substances, in order thereby to bring about a selective change in property through the contact with the product to be determined.
Properties denote features of the adsorbent which are changed by the contact with the product to be determined. Amongst others, these included optical properties, especially in the infrared, visible or ultraviolet region of the spectrum, i.e. colour changes which can be traced back to the contact of the product with the adsorbent, such as an emergence of colour or a decolourisation, the weight of the adsorbent, the temperature, the fluorescence, the refractive index, the pH value or the conductivity.
The change in property occurring through the contact with the product to be determined must be ascertained according to the invention. Depending on the property, this can take place for example visually or by measurement. It is necessary here to adapt the measurement method to the change in property. A temperature change of the adsorbent that is based on a specific exothermal reaction can for example be determined by means of an infrared camera. The determination can also be automated in the majority of cases.